Sunday, May 15, 2016

During WWII
the US State Department used several cryptosystems in order to protect its
radio communications from the Axis powers. The main systems used were the
unenciphered Gray and Brown codebooks along with the enciphered codes A1, B1,
C1, D1 and the new M-138 strip cipher.

In the period
1940-1944 German, Japanese and Finnish codebreakers could solve State
Department messages (both low and high level) from
embassies around the world. The M-138-A
strip cipher was the State Department’s high level system and it was
used extensively during that period. Although we still don’t
know the full story the information available points to a serious
compromise both of the circular traffic (Washington to all embassies)
and special traffic (Washington to specific embassy). In this area there was
cooperation between Germany, Japan and Finland. The German success was made
possible thanks to alphabet strips and key lists they received from the
Japanese in 1941 and these were passed on by the Germans to their Finnish
allies in 1942. The Finnish codebreakers solved several diplomatic links in
that year and in 1943 started sharing their findings with the Japanese. German
and Finnish
codebreakers cooperated in the solution of the strips during the war,
with visits of personnel to each country. The Axis codebreakers took advantage
of mistakes
in the use of the strip cipher by the State Department’s cipher unit.

Erich
Hüttenhain, who was the chief cryptanalyst of OKW/Chi (Signal Intelligence
Agency of the Supreme Command, Armed Forces), said in his unpublished
manuscript ‘Einzeldarstellungen aus dem
Gebiet der Kryptologie’, p20-21 about the strip system:

During the Second World War and until
September 1944, the United States used, together other cipher procedures, an
encryption system in its Diplomatic Service, Army, Air Force and Navy that was
known as the "Strip procedure."

The diplomatic strip system was the
most elaborate and equipped with the most variation possibilities. That is why
we will concentrate only on this procedure.

The strip procedure in the diplomatic
service was an encryption system for regular, scheduled services; but it also
allowed for sending broadcast messages from headquarters.

Almost every U.S. mission abroad had a
set of 50 strips for encrypted traffic with Washington, on each of which a
different scrambled alphabet was recorded twice in succession. Daily 25 of
these rods were selected and used in a prescribed order for ciphering: the
daily key.

After a few months the whole set of 50
strips was replaced by another set of 50 strips. In addition, every mission of
the US State Department had yet another set of strips for decrypting broadcast
messages from Washington. Also this broadcast set was changed from time to
time.

From a cryptologic point of view the
strip system was a periodic substitution cipher.

Soon after starting the investigations
of this encryption procedure we succeeded in identifying the period of 25. We
also succeeded to partially decipher some extra long cipher texts. It would
have been a hard and tedious work, if not some special circumstances would have
occurred. One of these circumstances was the fact that when the United States
changed a set of strips at one site it would reuse the same set at another
site, instead of cancelling it.

A second fact that was favorable for
deciphering was that, because of the U-boat blockade, it was not always possible
to replace the strip sets in timely manner at all the field offices. In such
cases, e.g. a broadcast message, for a site where the new broadcast strip set
had still not arrived, was encrypted with the existing special strip set at
that office and which had been in use for a long time.

If now this special procedure was
solved - and this was usually the case - the plain text of the broadcast
message was known, and there was a clear-cipher text compromise in the new
broadcast, from which the strips of the new broadcast procedure were
reconstructed.

In this way, a total of 22 different
lines and all broadcast messages were read from 1942 to September 1944.

From this successful decipherment must
be concluded:

1. It is prohibited to use a key
already used in other places.

2. It is not permitted to employ
encryption procedures that are vulnerable to clear text-cipher text
compromises.